CSS background image from PHP file

I’m trying to set a background in CSS using a PHP file like background: url(‘images/header-pics/display_header-pics.php’) no-repeat;

It works when I use:

<?php
$img = “3.png”;
header (‘content-type: image/png’);
readfile($img);
?>

But if I add a conditional statement nothing is returned. My code is:

<?php
if(is_single() || is_archive())
{
$imagePath = “3.png”;
}
elseif(is_home())
{
$imagePath = “2.png”;
}
else
{
$imagePath = “1.png”;
}

header (‘content-type: image/png’);
readfile($imagePath);
?>